Today, protection of the environment is of great concern to mankind. An ever increasing population as well as a general demand for increased quality of life expressed as a healthy and beautiful environment and at the same time a life style based on the use of advanced technology has accentuated the need for water, especially pure water throughout the world, but especially in the industrialized parts of the world.
In highly industrialized countries, especially countries with large urban concentrations, it is necessary to treat the waste water from households and industrial production so as to avoid an unacceptable level of polluted and polluting material in the environment, i.e. in the recipients for the waste water such as lakes, rivers and other waterways, the sea, etc. The polluted and polluting material comprises a variety of substances, for example organic and inorganic substances which may or may not be decomposable in nature. Among the polluting material usually present in waste water effluents decomposable organic matter and heavy metals are of the greatest concern.
An increasing amount of the waste water, which is produced worldwide, is now subjected to some kind of treatment, such treatment being of mechanical, chemical or biological nature or any combination thereof. Generally, it is expected that there will be focused even more on waste water treatment in the future as the public awareness of environmental hazards is becoming even stronger than today.
The main purpose of purifying municipal and industrial waste water is to reduce the content of biodegradable material in the waste water, i.e. to secure that the treated waste water does not contain such amounts of biodegradable material, i.e. degradable organic and/or inorganic matter, that these amounts will lead to an unacceptable low level of oxygen in the recipient due to the amount of oxygen required for aerobic decomposition of degradable (organic) material.
In order to fulfil this purpose, it is most desirable to be able to control the various steps of the purification process. As mentioned above, these steps are typically mechanical and/or chemical and/or biological, the use of biological treatment steps in a waste water purification process usually being the most sensible part of the overall process. Today, many waste water treatment plants, especially such plants comprising biological treatment steps, use some kind of process control. Generally, process control of industrial processes is based on knowledge of one or more of the most important parameters of the process in question, such knowledge being available on-line as a measured variable of the process, the controlled variable(s) being regulated based on this on-line information about the values of the measured variable(s).
As outlined above, the most important parameter in the treatment of municipal and industrial waste water is the content, i.e. the amount and the quality, of biodegradable material. In the field of waste water treatment this parameter is conventionally measured in terms of Biochemical Oxygen Demand (BOD). BOD is a measure of the amount of oxygen required for aerobic decomposition of organic matters. Biochemical Oxygen Demand evaluates the oxygen demand of the microorganisms performing the decomposition. However, it is disadvantageous that it is necessary to perform the BOD analysis on a sample under laboratory conditions and, furthermore, the BOD analysis does not give any precise information about the potential energy available as the microbiological degradation of the biodegradable material proceeds.
Thus, the state of the art is that the actual value of the most important parameter of a waste water purification process can neither be determined or monitored on-line nor be used as a measured variable in the process control system. Up till now, the various kinds of process control in the field of waste water purification have been based on measurements of parameters such as volumes, flow rates (residence time), pH, content of oxygen and/or of suspended solids in the waste water and the like. Especially purification processes comprising a biological step are extremely difficult to operate on the basis of the information about the state of the process which is available at present since the mixed cultures of microorganisms responsible for the decomposition of organic and/or inorganic matter, i.e. the biodegradable material present in the waste water, are most vulnerable to variations in the biodegradable material loading of the step. As a result thereof, the operation of such waste water purification plants are almost solely based on empirical knowledge of the chemical and/or biological processes which are actually taking place in the various process steps.
Also, it is a disadvantage of utmost importance that it is impossible to obtain on-line information about the quality of the final effluent from a waste water purification.